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<h1 style="margin-left: 15.0px;">Problems</h1><ol><li>Want better parity with JVM Clojure</li><li>async Javascript (ie. nearly all Javascript) makes the binding macro effectively useless without bound-fn</li><li>Some common Clojure behaviors, like printing to a dynamically bound *out*, can't currently exist across an async boundary. See *print-fn* in CLJS</li><li>I'd like to create a dom manipulation library that has a *dom* dynamic variable, which acts like current working directory for dom manipulation functions. I've got some crazy ideas that I want to experiment with here, especially once I implement add-watch: I think I can achieve pretty seamless UI data binding.</li></ol><h2 style="margin-left: 15.0px;">Design</h2><ul><li><strong>Are Vars still useful without threads?</strong><ul><li>Yes! Async callbacks and threads have a lot of common design considerations</li><li>Several interactive development use cases<ul><li>Vars are invokable, so you can preserve the indirection with the #'var special form<ul><li>(comp f #'g) ; returns a function that dynamically updates with g, but not with f.</li></ul></li><li>IWatchable can be used to instantly re-test a function in a live browser</li></ul></li></ul></li></ul><ul><li><strong>Performance</strong></li><ul><li>Price is equivalent to that of JVM Clojure</li><ul><li>Extra indirection for def and deref</li><li>Shared stack of dynamic binding frames</li><li>Hash lookup on each access</li></ul><li>Opt-in price for the Var indirection</li><li>Treat ^:dynamic as that opt-in mechanism; no impact for static vars</li><li>Potential optimization: Leverage Javascript's prototypical inheritance instead of Frame type</li></ul><li><strong>Required compiler analysis</strong></li><ul><li>Metadata for resolved vars</li><li>Not available for external libraries</li><ul><li>OK, because we only care about ^:dynamic vars</li></ul></ul><li><strong>Interop</strong></li><ul><li>Vars declared as ^:dynamic differ from static ones: they are wrapped in the Var type</li><li>binding, etc are only applicable to dynamic vars, not useful to non-ClojureScript callers</li><li>External callers can still use deref and alter-var-root</li></ul></ul><ul><li><strong>Impact</strong></li><li style="margin-left: 15.0px;">Breaking change for binding macro</li><ul><li style="margin-left: 15.0px;">New behavior matches JVM clojure: binding only works on dynamic vars</li><li style="margin-left: 15.0px;">Simply marking any affected vars as ^:dynamic should be enough to upgrade</li></ul><li style="margin-left: 15.0px;">Potentially breaking changes for any cljs.core vars that are changed to ^:dynamic</li><ul><li style="margin-left: 15.0px;">*print-fn* and other printing vars</li><li style="margin-left: 15.0px;">Only breaking for Javascript interop usages, still source compatible</li></ul></ul>